One of the special moves in the game of Chess is this en passant. It would definitely not be an exaggeration if I say that the en passant rule is probably the last rule most chess players learn, and some players never learn it.
The en passant move is a special move involving only pawns, and stems from the fact that pawns have the option to move either one or two squares on their initial move. If a pawn moves two squares in its first movement, but passes an enemy pawn on the 5th rank while doing so, the enemy pawn will have the option, if not the obligation, to capture that pawn.
Let me explain it to you in some detail, with an example.
Your opponent, playing black pieces, has a pawn on his second rank, while you have a pawn on an adjacent file on the fifth rank. Say, your opponent has a pawn in front of his knight on the queenside – that is at “b7”, and you have your pawn at “c5”. Assume that your opponent has moved the knight pawn a square forward to “b6”. Now you have the option of capturing the black pawn using the pawn at “c5” and this is a normal pawn capture, if you opt to capture.
Now, instead of moving one square ahead, the opponent’s pawn in front of Knight at “b7” chooses to move two squares forward, to “b5”. Now, you have the option of capturing that black pawn at “b5” en passant, which means “in passing”, as if the black pawn has advanced only one square and you captured the pawn in the normal course by moving your pawn to “b6”, capturing the opponent’s pawn at “b5”. The capture takes place as though the pawn had moved one square only.
This is all about en passant in Chess parlance, and commonly referred to e.p. in chess notation.
However, this en passant cannot be executed at your free will during any point of game. This special move can take place only for capturing pawns of the opponent using your pawns. This move will not be applicable to other pieces capturing Pawns, or pawns capturing other pieces. Though the en passant is optional only and not compulsory, it can be done only in reply to the move, on which the opponent’s pawn advances from the second rank. If the option is not taken at once, the option lapses irrevocably.